Eating something seasoned with Szechuan pepper is a pretty weird experience for the unaccustomed palate. After a few minutes, a tingling feeling comes on in the lips and tongue, often accompanied by a burning or numbing. Now, a group of researchers has described just what that tingling sensation feels like and named a particular kind of nerve fibre they think is involved.
Szechuan pepper is used in certain East Asian cuisines. For example, in Japan, Szechuan pepper is sprinkled on top of eel. "That buzzing situation with a burning situation is quite interesting," says Nobuhiro Hagura, a cognitive neuroscientist at University College London and one of the authors of the new study. "I like it," says Hagura, a native of Japan.
Hagura is interested in understanding how people perceive information transmitted from the skin. From reading the literature, he knew Szechuan pepper might activate a particular group of nerve fibres, known as RA1 fibres, which respond to light touch and vibration. Vibration receptors help tell you whether, for example, you're running your finger across silk or denim. Hagura and his colleagues wanted to know whether Szechuan pepper set off those receptors.
In a series of experiments, the researchers put ground-up Szechuan pepper on the lower lips of volunteers, as they describe this week in the Proceedings of the Royal Society B. All the participants agreed the pepper created a tingling sensation; most also felt burning or numbing. In one experiment, the researchers put pepper on the lip and at the same time stimulated the person's finger with a mechanical vibrator that could be slowed down or sped up. The person judged when the frequency of tingling on the lips and in the finger was the same.
The researchers found Szechuan pepper tingled at an average frequency of 50 cycles per second. That's the frequency of vibration that RA1 fibers are most sensitive to.
Neuroscientists have used menthol and chili peppers to help them understand how we sense pain and temperature, and Szechuan pepper could be useful for understanding the mysteries of touch, Hagura says. "If you grab a cup, it sounds like a very simple act," he says. "In reality, what you feel from grabbing a cup is many signals from many fibres." Szechuan pepper might help neuroscientists by giving them a tool that can create the sense of touch in the lab.